(1) Field of the Invention
The present invention relates to a polyester fiber having a high strength, a low shrinkage, a good fatigue resistance and a good drawablity, which is valuable as an industrial material.
(2) Description of the Prior Art
Polyester fibers are broadly used not only as a textile material but also as an industrial material because they have various excellent characteristics. Especially, a polyester fiber having a high strength and an excellent dimension stability is very valuable as an industrial material and is used for the production of tires and other industrial products. However, higher properties are recently required for polyester fibers. For example, in the field of the production of the conveyor belts and rubber hoses, a much reduced shrinkability is required in view of the dimension stability at the modeling step and it is required to impart durability and fatigue resistance capable of resisting severe application conditions to starting polyester fibers. Furthermore, in the field of tire cords, in order to improve the yield at the tire-forming step, it is required to further reduce the shrinkability, and in order to improve the driving comfortability, it is required to increase the modulus. Moreover, in the application to large-size tires, improvement of the fatigue resistance is desired, and in case of cords for V-belts, increase of the modulus is required to attain a maintenance-free effect and in case of cords for large-size high-load wrapped belts, high elongation, high toughness and high fatigue resistance are required. In view of the foregoing, it is considered that if there is provided a polyester cord having a high strength, a low shrinkability, a high modulus and a high fatigue resistance in combination, the superiority of the polyester fiber to other materials in the manufacturing cost will be increased and the application field will be further broadened.
The polyester fiber is inferior to older rayon and Vinylon fibers in the modulus and shrinkability and is much inferior to older general-purpose polyamide fibers in the fatigue resistance, and it is important to improve these insufficient properties. If these poor properties are improved, the superiority of the polyester fiber to rayon, Vinylon and polyamide fibers in the cost and performance will be increased and the polyester fiber will be more highly evaluated as in industrial material.
As means for realizing a high strength required for an industrial fiber, there is known a process in which a polyester having a high degree of polymerization is used, the molecular orientation is controlled at the spinning step and the drawing ratio is increased to a level as high as possible at the drawing step, as disclosed in Japanese Examined Patent Publications No. 41-7892 and No. 53-1367. According to this process, however, a polyester fiber having high strength and toughness can be obtained but a polyester fiber having low shrinkage can hardly be obtained. As means for reducing the shrinkability, there is known a process in which a polyester fiber is heat-treated at a high temperature under a low tension after multi-staged drawing, as disclosed in Japanese Examined Patent Publication No. 58-51524. However, this process is defective in that only a fiber having a lower fatigue resistance is obtained as in the above-mentioned known process.
As means for improving the fatigue resistance while maintaining the shrinkability at a low level, for example, Japanese Unexamined Patent Publications No. 53-58031 and No. 53-58032 propose a process in which the molecular orientation of a drawn yarn of a polyester fiber is reduced and the work loss is reduced. This process is characterized in that a polyester fiber is rapidly cooled just below a spinneret by a gas atmosphere maintained at 10.degree. to 60.degree. C. However, since drawing is performed to the point just before the point of breakage so as to increase the strength, the elongation is extremely low, and since yarn breakage frequently occurs at the drawing step, the process is defective in that stable production is difficult.